The present invention relates generally to transparent polyurethane compositions, and to coated and laminated aircraft transparencies incorporating such compositions, and more particularly to such compositions, coated transparencies, and laminates having antistatic or static dissipative properties.
Transparencies used for modern military aircraft often require a protective, erosion-resistant coating or film located on the outer surface. Such protective outer layers are required to prevent damage to fragile underlying metal or ceramic conductive coatings, such as gold or indium tin oxide (ITO), or to protect a plastic surface having limited environmental durability, such as polycarbonate. Transparent polyurethane coatings and films are preferred for these applications due to their superior erosion resistance properties, excellent transparency and good environmental durability.
The outer surface of an aircraft transparency is subject to electrostatic charging, especially with high performance aircraft. This charging is caused by contact with ice crystals and other particles during flight, which results in transfer of a charge to the surface via triboelectric or frictional effects. This phenomenon is called precipitation charging, or p-static charging, in the industry.
P-static charging of a non-conductive (dielectric) outer surface can create several serious problems affecting aircraft performance, transparency service life, and safety. Discharge during flight can result in damage to outer coating layers from dielectric breakdown or can result in electronic interference with instruments. Such charge accumulation can also create shock hazards for flight and ground personnel.
To prevent these problems caused by charging, the outer layer of an aircraft transparency must be sufficiently conductive to allow the charge to drain across the surface to the airframe or through the thickness of the layer to an underlying conductive metallic or metal oxide layer. Polyurethanes and other organic polymers generally are poor conductors of electricity. Consequently, these polymers cannot be used satisfactorily without modification in applications where static dissipative properties are required.
Several methods have been used in the past to modify polyurethanes so as to increase their electrical conductivity, and thereby to better dissipate a buildup of static charge. In one such method, conductive fibers or particles are incorporated into the polyurethane matrix. This method is not suitable for use with polyurethanes that are transparent, however, because the conductive filler materials are opaque and greatly reduce the light transmission of the modified material. With a relatively thick layer required for highest erosion resistance, >0.002 inch (>50μ), incorporation of such additives to required levels reduces light transmission drastically.
In another method for modifying polyurethanes to increase their electrical conductivity, conductive polymers such as polyaniline or polythiophene salts are incorporated into the polyurethane matrix. Again, however, this method is not suitable for use with polyurethanes that are transparent, because the conductive polymer additives form a dispersed phase that reduces transparency. In addition, polyanilines, polythiophenes and other conductive polymers do not have good environmental stability and generally cause reduction in overall resistance to weathering and environmental degradation.
In yet another method for modifying polyurethanes to increase their electrical conductivity, hydrophilic additives such as amines and quaternary ammonium salts are used to increase the polyurethane's surface conductivity. These additives function by migrating to the polyurethane's surface, where they attract water and thereby create a conductive film. This method is not suitable for polyurethane coatings and laminates, however, because the additive also migrates to the surface of the polyurethane that interfaces with the underlying substrate, resulting in a loss of adhesion. In addition, such additives can lose their effectiveness over time, because they can leach from the polyurethane under normal use conditions.
In general, non-ionic additives and polyol modifiers have been found to significantly enhance electrical conductivity only if used at high levels, which can adversely affect other important properties, such as transparency and mechanical strength. Ionic additives, including quarternary ammonium salts and ionizable metal salts, generally are more effective in enhancing electrical conductivity. The most effective known additives of this kind are ionizable metal salts of perfluoroalkylsulfonates. However, none of these ionic additives is considered fully satisfactory for use in transparent polyurethanes used as coatings or in laminates, because they are fugitive and with aging they can cause a loss of transparency and a loss of adhesion.
None of these known additives for increasing the electrical conductivity of polyurethanes is considered fully satisfactory for use in polyurethanes that are transparent, and particularly in polyurethanes that are used as coatings or in laminates for aircraft windows.
Recently, systems containing less fugitive additives or modifiers have been developed that show improved performance compared to dispersed salts. Such systems are described in U.S. Pat. No. 6,458,875 to Sandlin et al. Additional performance improvements beyond those disclosed in U.S. Pat. No. 6,458,875 have been demonstrated by use of ionic functional groups incorporated into the polyurethane polyol backbone to create a truly non-fugative ionic system. These modifications represent the current state of the art.
All prior art modification methods used to enhance conductivity of thick polyurethane coatings and films suffer from one limitation: Conductivity is strongly dependent on temperature. As temperature is reduced, conductivity and ability to dissipate accumulated surface charge drop by orders of magnitude. Since aircraft operating at high altitudes regularly experience transparency surface temperatures of −40° F. or lower, electrostatic discharge capability must be maintained at low temperatures. Current systems are marginal with respect to this requirement.
It can readily be appreciated that there is a need for a protective, thick polyurethane coating that maintains high p-static dissipation capability at all operational temperatures without compromise of other performance properties. The present invention fulfills this need and provides further related advantages.